This invention relates to a process for removing polymer scale adhering to a polymerization vessel.
Most of vinyl chloride polymers are prepared by polymerizing vinyl chloride monomer alone or a mixture of the vinyl chloride monomer and other monomer in an aqueous medium. More particularly, an aqueous medium is at first charged into a polymerization vessel, then vinyl chloride monomer is added thereto together with a polymerization catalyst to form a dispersant of the vinyl chloride alone or the mixture thereof in the aqueous medium, and a polymerization reaction is carried out in the dispersant to obtain a vinyl chloride polymer.
After the polymerization reaction has been carried out in the manner as mentioned above, the resulting vinyl chloride polymer is discharged together with the aqueous medium from the polymerization vessel. At this time a polymer-like substance usually referred to as "scale" is found as adhering to the inner surface of the polymerization vessel. When the polymer scale adheres, it causes not only the polymerization yield to be decreased, but also hinders conduction of heat on the surface of the vessel, and it is difficult to control the temperature in the polymerization vessel, accordingly the polymerization reaction cannot be advanced as desired. Furthermore, it sometimes happens that the scale is peeled off from the inner surface of the vessel and mixed into the polymer to be the product. When the scale is mixed into the product, it gives rise to fish eyes in fabricated articles and deteriorates the quality of the articles. Adhesion of the scale brings about such disadvantages as mentioned above, and it is necessary to prevent the scale adhesion when the polymerization is carried out, and in fact efforts have been heretofore made to remove as much as possible the scale adhering to the vessel.
In order to minimize the amount of the adhering scale, attempts have been made to apply a specific compound beforehand to the inner surface of the polymerization vessel. When the specific compound is applied, it is true that the amount of the scale is decreased, but it cannot be so much decreased as desired. Furthermore, if the scale is formed on a certain portion of the inner surface of the vessel even in a small amount, the scale tends to accumulate voluminously further in the same portion. Moreover, the scale has the characteristic that it is liable to be formed on the portion of the inner surface wherein the surface of the vessel is marred. Accordingly, after the polymerization had been carried out, it was necessary to remove the scale as completely as possible so that the polymerization vessel may not be marred.
In order to meet this necessity several methods have hitherto been proposed. One method is that a man enters into the polymerization vessel and scrapes off the scale adhering to the vessel by human power and by means of tools such as a scraper. Another method is that a highly pressurized water is jetted onto the inner surface of the vessel to wash away and remove the scale by the water stream. Still another method is that the scale is dissolved in an organic solvent and discharged together with the solvent. These methods, however, have advantages on one side and disadvantages on the other side and cannot be satisfactory. More particularly, the method by means of the human power tends to mar the inner surface of the vessel and thus results in promoting formation of the scale. The method by means of the pressurized water tends to remain the scale partly on the surface and cannot remove the scale completely, thus results in that the scale is accumulated in the vessel to the contrary. The method using the solvent tends to form a polymer film on the inner surface of the vessel, and thus helps formation of the scale.
Among the methods using the organic solvents, there is a specific method in which the organic solvent is mixed with water and a surface active agent and the resulting mixture is used for dissolving the scale. The method is disclosed in the Japanese Examined Patent Publication No. 47-19896. The method requires that a polymerization vessel be filled with the organic solvent containing water and the surface active agent, and that the resulting mixture be agitated in the vessel. Thus the method requires use of a large amount of the organic solvent and the method is not only uneconomical, but also it takes a long time for removing the scale because the organic solvent is mixed with water and is decreased in dissolving power. Furthermore, the method uses the surface active agent, which tends to form and adhere the scale to the vessel when the subsequent polymerization is carried out, still further some surface active agent tends to cause the subsequent polymerization to advance in abnormal manners and brings about various disadvantages.